A polyester resin, for example, polyethylene terephthalate is excellent in mechanical strength, heat resistance, transparency and gas barrier property, and suitably used as a material for containers used to package beverages such as juice, refresh beverage and carbonated drinks and the like, as well as a material for films, sheets and fibers.
Such polyester resin is usually prepared using dicarboxylic acid such as terephthalic acid and the like, and aliphatic diol such as ethylene glycol and the like. Specifically, first, aromatic dicarboxylic acids and aliphatic diols are subjected to esterification to form a low molecular condensate (ester oligomer), then the low molecular condensate is subjected to deglycol reaction (liquid polycondensation) under presence of polycondensation catalyst to attain a high molecular weight of the polymer. In addition, the polyester resin which is used as a material for a beverage packaging container, is usually prepared by carrying out solid polycondensation followed by elevating the molecular weight with volatizing and removing low molecular side products such as acetaldehyde which adversely affects the taste of the beverage. This polyester resin is then provided to a molding machine such as an injection molding machine to form a preform for a hollow molded body. Thereafter, the preform is inserted to a mold having a certain shape for stretch blow molding, or further heat-treated (heat set) to form a hollow molded container.
In such method for producing the polyester resin, an antimony compound, a germanium compound and the like are conventionally used as a polycondensation catalyst.
However, the polyethylene terephthalate produced by using antimony compound as a catalyst is inferior to that produced by using a germanium compound as catalyst in respect to transparency and heat resistance. On the other hand, the high cost of germanium compound increases production cost of the polyester resin. To reduce the catalyst cost, there needs a process such as recovering and recycling the germanium compound volatized in the polycondensation.
In addition, since polymerization activity per metal weight of the antimony compound or germanium compound and the like is not high, the use of relatively high concentration of the antimony compound or germanium compound or the like is needed to produce a polyester resin at a rate to satisfy industrial production. As a result, the polyester resin produced by using these compounds usually has 50 ppm to 300 ppm of antimony or germanium and the like as a metal atom.
In recent years, in view of the impact the industrial products have on the earth environment from their production to disposal, it is strongly required to reduce their adverse effect should be reduced. For example, when considering the life cycle of the polyester products as a beverage packaging container, it is important to minimize metal, in particular, heavy metal from being flown out from the polyester container to the beverage. Hence, it is preferred that the metal content in the polyester resin is low. Further, a low metal content is also preferred in burning up the polyester resin after use since the metal is a source of ash which needs additional treatment. In addition, the low metal content is also preferred in depolymerization of the polyester resin to recover and recycle the monomers after use since the metal may be a source of the impurities in the recovered monomers. As described above, reducing the content of the metal, in particular, heavy metal contained in the polyester resin has significant meanings.
By the way, titanium is known as an atom to have activity to promote the polycondensation reaction of the low molecular condensate. Titanium alkoxide, titanium tetrachloride, titanyl oxalate, orthotitanic acid and the like are well-known as a polycondensation catalyst. Many investigations have been conducted to use such titanium compounds as a polycondensation catalyst. Such titanium compounds have high polymerization activity per metal weight, and are catalysts which may reduce the amount of metal to be used, considering only the aspect of the production rate of the polyester resin. In other words, the titanium compounds may be used usually in an amount of several ppm to 50 ppm in terms of converted titanium atom in producing the polyester resin using these compounds.
Although these titanium compounds have the high polycondensation activity per metal weight, they have a strong tendency to cause undesirable polyester decomposition reaction, and cause the resin quality to be deteriorated by coloring the resin in the polycondensation process by producing the side products having low molecular compounds or by decreasing the molecular weight and the like in the melt molding process.
As a result, the polyester resin produced by using these titanium compounds as a polycondensation catalyst has low stability, and acetaldehyde produced by thermal decomposition at the time of melt molding and decrease of the molecular weight, are more prevalent than those of the polyester resin produced by using the conventional antimony compounds or germanium compounds and the like as a polycondensation catalyst. Therefore, at present, the problems still remain in using the polyester resin produced by using the titanium compounds as a polycondensation catalyst, as a material for a beverage packaging container.
On the other hand, if the amount of the above-described titanium compounds is reduced so as to decrease the deterioration of the resin quality caused by thermal decomposition at the time of melt molding the polyester resin, the polycondensation rate of the polyester resin becomes lower than that of the polyester resin produced by using the conventional antimony compound or germanium compound and the like as a polycondensation catalyst. As a result, longer polymerization time or higher polymerization temperature is required, which increases the production cost of the polyester resin.